Solid state starters/controllers have found widespread use for controlling application of power to an AC induction motor. The conventional starter/controller, referred to hereinafter as simply a starter or a controller, uses solid state switches for controlling application of AC line voltage to the motor. The switches may be thyristors such as silicon controlled rectifiers (SCRs) or triacs.
One application for a motor controller is as an elevator starter. The elevator starter may be used to drive a pump for an hydraulic elevator. Each time movement of an elevator car is commanded, then the starter must start the motor until it reaches operating speed and then operate in a run mode. Such a starter may only be used for the up direction as gravity may be used for the down direction.
A conventional motor controller system may use current transformers for sensing current of each motor winding. A standard overload system determines if sensed current exceeds a preselect threshold amount for a specified amount of time and if so either halts motor operation or decreases current supplied to the windings by varying control of the solid state switches. Elevator systems typically use a delta motor. The controller switches are wired inside the delta. Delta motors may be imbalanced. Conventional control systems use average current through the windings to determine overload conditions. However, this could result in damage to a single winding if, for example, the single winding has an excessive current and the remaining winding currents are within limits so that the average current is less than the overload setting. Known motor controllers that measure imbalance among the winding currents use individual measurements to determine if an imbalance condition exists. However, imbalance is determined by comparing measured currents to a fixed trip level.
The present invention is directed to solving one or more of the problems discussed above, in a novel and simple manner.